Found a manual (meaning outside of standard config / package) and hacky workaround, would love to hear of any improvement over that :)
Create a user in pfsense's User Manager, enable SSH access for that user with a password-less SSH key login (I'm aware it's risky, extra precautions below).
Create a script in the home user dir, show_wan_ip.sh, containing:
#!/bin/sh
ifconfig mvneta0.4090 | sed -n '/.inet /{s///;s/ .*//;p;}'
Edit ~user/.ssh/authorized_keys and add the following before the key:
command="/home/user/show_wan_ip.sh",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty
This can be executed from the (less trusted) PC that connects to it over LAN:
ssh user@10.100.1.1 "/home/user/show_wan_ip.sh"
192.168.1.10